In the ceramics field, digital printing decoration technology, generally through the use of ink-jet printers, has become established in the last decade. Digital printing has allowed a marked improvement in the aesthetic qualities of ceramic products, making it possible to produce decorations with extremely complex patterns, such as for example floor or wall tiles that reproduce the appearance of natural materials. The inks used in digital printing devices generally consist of a solvent or a mixture of solvents and one or more finely ground pigments dispersed in the solvent. Although they allow surface decorations to be produced, these inks are unable to penetrate the ceramic material onto which they are applied as they contain suspended solids, therefore the decorated surface must be covered with a protective layer, generally a transparent glaze (known as “crystalline” in the field), to prevent discolouration of the decoration due to mechanical wear and attack by both chemical an mechanical atmospheric agents. This makes producing unglazed ceramic materials using this type of inks impossible. Inks containing, in addition to the solvent, also completely soluble organic compounds of chromophore metals, can be used to produce unglazed products coloured also within the ceramic mass. These chromophore metals give rise to development of the colouration during firing of the coloured manufactured articles. In digital printing processes the desired colour is normally obtained by the subtractive mixing of the colours of the inks comprising the print set. Each ink set will therefore only be able to produce a certain range of colours (known as gamut) that represents a more or less broad portion of the colorimetric space: the broader the gamut produceable with a given ink set, the more colour tones are obtainable with that set. To obtain a sufficiently broad gamut, an ink set generally comprises an ink that is able to develop the colour yellow after firing.
It is known in the field that colourant solutions containing soluble compounds of chromium in combination with soluble compounds of Sb, Zn, Zr and/or Mn can be used to obtain the colour yellow in a colouring method of ceramic supports to which titanium dioxide is added (EP 894081). Alternatively, colourant solutions containing soluble compounds of chromium in combination with soluble compounds of Sb and/or W and soluble compounds of Ti can be used, said solutions being able to develop after firing the colour yellow on ceramic materials not mixed with a titanium dioxide additive (EP 940379). The simultaneous presence of chromium, antimony and titanium in the coloured portion of the ceramic material is therefore an essential requirement for development of the colour yellow after firing. The hue and saturation of the yellow obtained will largely depend on the quantity ratio of these chromophore metals in the coloured portion of the ceramic slurry.
Patent EP 1272574 describes a CMY ink set for digital printing on glazed ceramics based on organic solvents, wherein one of the inks useful for obtaining the colour yellow contains a mixture of soluble complexes of antimony and chromium and/or nickel, in combination with a source of titanium. According to one of the variants described in this patent, titanium is present in the ink in the form of a soluble compound, preferably titanium tetraisopropoxide (paragraph [0050]) or as a compound derived from another alkoxide (Example 4, paragraph [0075]). In the printing process, this ink can be indifferently applied to a conventional glaze or to a glaze mixed with an additive to which titanium dioxide has been added to intensify the development of the colour yellow.
Patent application WO 2009/077579 describes an alternative ink set for ceramic ink-jet printing wherein the yellow colour is obtained with an ink (composition C2) containing an organic compound of titanium in addition to a soluble organic compound of chromium or nickel in combination with a soluble organic compound of tungsten or antimony. In the case of an organic solvent-based ink, this organic compound of titanium is preferably titanium 2-ethylhexanoate. The described ink set is used in a digital printing method on ceramic materials mixed with titanium dioxide and amorphous silica as additives.
Although the inks for obtaining the colour yellow described in the previous patents can be used for ceramic digital printing, the titanium derivatives contained in these inks are extremely water sensitive and hydrolytically unstable compounds and easily give rise to precipitates on contact with water, which is generally present with a content of about 5% by weight in the unfired ceramic tiles; since these tiles have temperatures between the ambient temperature and 60-70° C. at the time of decoration, the water contained in the tiles evaporates during printing, ascends the conduits of the printing heads from which the inks are expelled and comes into contact with the inks themselves before these are printed, causing the precipitation thereof within the print devices. Consequently, use of the inks described in the previous patents leads to frequent clogging of the printing head nozzles and to a significant reduction in line production capacity due to the need to perform frequent maintenance, cleaning and replacement operations of the same printing heads.
Titanium derivatives poorly sensitive to water, such as those described in U.S. Pat. No. 2,643,262 and GB 1586671 have been known in the field for some time.
U.S. Pat. No. 2,643,262 describes organic titanium derivatives, called “glycol titanates”, obtainable from the reaction of a titanium alkoxide and a 1,3-diol in titanium:diol molar ratios variable between 1:0.5 and 1:4. These titanium derivatives are generally complex monomers or polymers and can be used as adhesives, surfactants and additives. In Example 1, the patent illustrates the synthesis of a glycol titanate by reacting equimolar amounts of titanium tetraisopropoxide and 2-ethyl-1,3-hexanediol, in the presence of n-heptane as solvent.
GB patent 1586671 describes analogous titanium derivatives, to be used as transesterification catalysts, which can be obtained by reacting a titanium alkoxide and a 1,3-diol in Ti:diol molar ratios of between 1:1 and 1:2. In the description of the preparation of the catalyst J, titanium tetraisopropoxide and 2-ethyl-1,3-hexanediol are reacted in equimolar amounts in the presence of petroleum ether as the solvent. The titanium derivatives described in this patent are insensitive to moisture. However, when used in a mixture with organic compounds of chromium for the formulation of inks to obtain the colour yellow in ceramic digital printing, said titanium derivatives are not always compatible with the organic compounds of chromium and can also form hydrolytically unstable mixtures with these.
There is therefore a continuing need in the field need to provide organic titanium derivatives useful for the preparation of inks for digital printing on ceramics that are able to develop the colour yellow during firing of the decorated material and that are fully soluble in organic solvents, hydrolytically stable and compatible with organic compounds of chromium and/or nickel.
Object of the present invention is to provide an organic titanium derivative that has the above-cited characteristics as well as to provide a process for the preparation of said derivative, a ceramic digital printing ink containing said derivative and a digital printing method on ceramics that makes use of said ink.